The field of invention pertains to coating electroconductive articles to produce colors thereon with a sense of depth and beauty for decorative purposes. In particular, the invention relates to electroplating of such coatings having decorative and functional utility for deposition on numerous electroconductive substrates.
There exists a need for a relatively inexpensive means of plating articles to provide vivid spectral interference thicknesses with greater coverage consistency and reproducibility than exist in the prior art.
It is a strong argument that the superior visual appearance of one commodity given the choice between competing brandsxe2x80x94is the main factor influencing a consumer""s final purchasing decision. Metallic coatings have historically been primarily paint based. Although, with chemical vapor deposition and similar technologies, metallic oxide coatings are finding several new functional and decorative applications. Such coatings can span a vast range of optical phenomenon: Metallic coatings exhibit luster. The coatings exhibit diaphaneity (i.e., they can be opaque or semi-transparent, or transparent). As such, the luster of the metallic coatings can be richly specular or present diffuse illumination from their reflective surfaces. They can display all manner of iridescence, including pearlescence and a more colorful nacreous look. Some metallic coatings exhibit goniochromism (dichromatic reflection) whereby a surface can display distinct color change depending on the angle of the viewer. Metallic coatings may also show temporal properties, such as glitter, glint, and aventurine brilliants. Such phenomena are related to the concept of binocular depth. Human visual perception can sense clarity in these films, as well as xe2x80x9cbinocular mottle.xe2x80x9d This term was coined by Calvin S. McCarnyxe2x80x94it is the sense of texture resulting from the reflection of a subsurface through a translucent metallic surface-film.
Prior Art.
Metallic paints suffer from the drawback that they are not as specular as brightly polished or bright electroplated metals. Furthermore, there are inherent difficulties in obtaining paint uniformity over large surface areas in terms of color, thickness and luster. For multiple color applications, there is a need to purchase several different paints, or, if possible, mix new colorant pigments. Many consumer items which are fashioned from metal are painted because of the cost associated with electroplating and the desire to have colorful objects.
The marriage of clear lacquers with electroplated work-pieces is used industrially to cover dissimilar metals, such as zinc die-castings and steel, in order to give the articles the same color tone and luster. In the prior art, it is known to take such work-pieces and apply finely divided bronze and copper metal powders, usually by spraying them on through a pneumatic spray jet in an admixture of lacquer. The painter then applies additional lacquer layers and polychrome dyes (known as polychroming) in order to achieve artistic results, such as an antique bronze finish. Once the article is painted, the underlying electroplated coating is partially visible, giving the article an increased specular metallic reflection. However, such work is tedious, labor intensive and involves both an artistic temperament and a technical understanding of the compatibilities of lacquer types and solvents, and there is the problem of color matching with the dyes.
The coloring of metals dates to ancient times. The discovery of Greek papyri manuscripts at Thebes in the early nineteenth century, believed to be from the sarcophagus of an ancient Egyptian alchemist, discusses, among other topics, the surface coloring of metals. [Lagercrantz, Otto. Papyrus Graecus Holmiensis, Upsula, Sweden: Akademiska, Bokhandeln, 1913]. There are a number of methods for depositing oxides on metal surfaces which imbue the metal substrate with light polarized interference colors. The discovery of this phenomenon is often attributed to the physicist Leopoldo Nobili (1784-1835), who conducted his experiments in 1826. He is credited with founding the art of metallochromy. However, the Edinburgh Philosophical Journal of 1823 discloses the inventions of Sir John Barton (1771-1834) on the production of xe2x80x9ciris metal.xe2x80x9d
A metallochrome is defined as a coloring produced by light polarization of a deposited metallic compound. The authors do not wish to be bound by theory. It is believed, however, that the metallic film is to some degree translucent, allowing the transmittance of visible light through the film whereby some of the light is reflected at the film""s exterior surface and whereby the remaining light is both partially adsorbed by opaque constituents within the semi-transparent film and reflected at the subsurface [substrate] layer causing the display of spectral colors by the optical interference of both surface and substrate reflections whose wavelengths correspond in hue to the thickness of the film. xe2x80x9cNobili""s ringsxe2x80x9d are a rainbowy formation created by the irregular deposition of peroxide of lead from a bath of lead acetate. [David Fishlock. Metal Colouring, Teddington, England: R. Draper, 1962]. Nobili also succeeded in the deposition of cuprous oxide from a bath of copper acetate.
Other pioneers in the art of metallochromy were Fechner and later Elsner, who used zinc compositions. Boettger used a bath of copper and ammonium chloride. M. Puscher used lead acetate and sodium hyposulphite (c. 1869).
The noted physicist Antoine Cxc3xa9sar Becquerel (1798-1878) also experimented with several elements and bath compositions. He is known as one of the founding fathers of the science of electrochemistry. He was the first to make the voltaic pile with constant current, thus ushering in the age of electrodeposition. His research included electroplating and electro-metallochromy.
After its introduction in 1845 at the German Industrial Exposition [Berlin], metallochromy found commercial application for use on toys and watch dials in factories in Nuremberg and Feurth, Bavaria. [xe2x80x9cMetallochromy,xe2x80x9d Manufacturer and Builder, Volume 1, (7), July, 1969, p. 210].
There is little evidence that aqueous bath electroplated and immersion metallochrome coatings have been used in industry after that time. This may be due to the craft tradition of trade secrets. It is believed that the commercialization of metallochrome finishes fail due to the difficult nature of achieving consistent results. The principal drawback of metallochrome coatings is that they were not very durable for most applications. An article of metallochrome jewelry, for example, would wear within a short time period and lose its beauty. Certain metallochromes are subject to galvanic reduction by metals. The coatings are also subject to chemical reduction by acids and lacquer coatings with acidic pH levels and to thermal reduction. Applications for the coloring of more exotic metals by means of light interference will be briefly reviewed:
Certain valve metals may be anodized to produce a dialectric coating exhibiting interference colors, see A. F. Torrisi, xe2x80x9cRelation of Color to Certain Characteristics of Anodic Tantalum Films, xe2x80x9cJournal of the Electrochemical Society, April 1955, 102, (4), pp. 176-180. The reactive metals titanium and niobium may be anodized to produce polychromic effects and is largely used for jewelry applications. Also, anodizing techniques for coloring aluminum by means of light interference are known to the art.
The coloring of zinc is largely accomplished by chemical conversion coatings. In industry, steel is electroplated or hot dip galvanized with zinc, and in order to improve its corrosion resistance, a conversion coating with a chromate containing trivalent or hexavalent chromium, and often silver, is used. The photo-active silver in such conversion coatings has the disadvantage that exposure to ultraviolet light will discolor and diminish the luster. It is costly, in terms of waste processing, to treat for chromium and silver.
A light polarizing colored conversion coating on nickel is know to the art: Reference Keping and Fang in the reference citations above. Their coating uses cathodic passivation treatment of electroplated nickel by immersion in a molybdate phosphate solution.
The applicants are aware of a decorative coating which is similar in nature to the foregoing invention: Gorodetski, et al are the inventors of International Publication No. WO97/43127 (henceforth ""127) xe2x80x9cArticles Having a Colored Metallic Coating and Processes for Their Manufacturexe2x80x9d (granted Nov. 20, 1997) assigned to Nickel Rainbow Ltd. (Yavne, Israel). This metallochrome coating is produced in a sulphide bath containing nickel, zinc, ammonium and thiocyanate ions when plated at current densities of 0.1 to 0.5 A/dM2.
Blackening of Metals.
The artificial blackening of metals generally began by placing artifacts, such as cooking ware, over stove fires or coals. In the field of decorating metals, the artisan wanted to create an attractive aesthetic effect or to simulate the natural oxidation of antiquities. Puscher discovered that zinc could be blackened by boiling it in a solution containing sodium hyposulphite, lead acetate and copper sulphate.
As previously stated, the conversion coating of zinc is subject to decoloring due to the reaction of the silver component in conversion coatings. A black conversion coating of zinc can likewise be decolored, turning it to a gray or olive drab. This is undesirable.
Liver of sulfur (or sulfurated potash) is another commonly applied immersion for blackening copper, brass and silver. Incidentally, the process will also deposit spectral metallochromy films on substrates. The substance is a metastable compound of potassium polysulfides and sulfate which absorbs water and carbon dioxide from the air. It therefore has a short bath life rendering it unsuitable for industrial use.
Excellent processes in terms of specular reflectivity incorporate black rhodium and rhenium. There are also processes known in the art to create a blue finish on rhodium. There are no known aqueous cathodic processes for producing spectral interference colors using platinum group metals (PGM""s). The primary disadvantage to coating applications using PGM""s is their expense. This is only slightly mitigated by the fact that such depositions can be electroplated with relatively thin thicknesses. Please reference U.S. Pat. No. 4,416,742, xe2x80x9cProcess and Electrolytic Bath for Making a Rhodium-plated Article Having a Black or Blue Color,xe2x80x9d issued to Takashi, et al. See also Japanese patent 57174485, xe2x80x9cBlack Colored Rhodium Plating Solutionxe2x80x9d (10/1982) issued to Toshiyuki, et al.
Alternative technologies for the deposition of thin oxide films include sputtering, chemical vapor deposition, physical vapor deposition and ion beam deposition. These processes require comparatively expensive high technology equipment.
The invention comprises electrodeposition of nickel oxide and nickel-zinc oxide electrodeposited metallic thin film coatings. The bath electrolyte is composed of sodium lactate and chloride constituents. The metal ions are composed of zinc and nickel. A thiocyanate is also added to the bath. The coating comprises various spectral colors, depending on the length of electrodeposition time in a bath at a fixed temperature and electric current density. The article can be partially removed from the bath to create colorful aesthetic effects. The selective patterning of the part with a stop-off material will also create polychromatic effects. A durable final clear protective coating, such as acrylic lacquer, epoxy enamel, polyurethane, or a thermally cured powder coating, is applied for permanent decorative finishes.
The primary advantages of the present inventors"" discovery vis a vis Gorodetski, et al., is that the alloy film bath which we have devised can operate in alkaline, neutral and acidic pH ranges. The fact that the bath can operate at an alkaline pH range makes it possible to deposit metallochrome films on a number of new substrates. Therefore, it is an object of the invention to provide a metallochromy alloy nickel bath which can be deposited on the so called difficult to plate substrates, such as aluminum, titanium, beryllium, leaded steels, stainless steels, etc. These substrates are noted for the fact that they deposit tenacious oxides quickly and therefore must be stripped of these oxides and pre-treated with more favorable metallic films, such as zincates.
Further, our invention will produce true black coatings, which is an embodiment specifically excluded in the ""127 patent. The ""127 patent also excludes white deposits. By modification of our bath to an alkaline pH level between 11 and 12, we have found that a whiteish translucent oxide of zinc can be electrodeposited on a substrate and that colorful metallochromy deposition ceases. A specific advantage of the high pH bath is that it offers an exceptional strike coating for the lower pH metallochromy embodiments in order to obtain color uniformity on large surface areas.
Another advance over the ""127 patent is that by polarizing (or anodizing) an article which has been plated with the nickel alloy metallochromy film at 0.01-0.05 amps per sq. inch, and then subsequently re-plated to the identical coating, colors of greater depth and deepened richness and greater color consistency may be achieved.
Another advance over Gorodetski ""127 is that there are three new methods to control the ability of the operator to match a color sample. These methods are: polarization (oxidation), reverse plating (which both strips and oxidizes the film), and reduction (which lightens or color-alters the film).
First, by reverse plating an article""s color may be changed. The intensity of the article may also be darkened by polarizing an article at 0.01 to 0.05 amps per square inch. The colored interference film of our invention can the be plated over the now color-altered or darkened coating. There are two ways of accomplishing this oxidation. One method is to use in situ power interruption. In this case, the bath will become a battery on its own in reaction to the article (which may have been energized by the prior plating). Thus, the coating on the part may partially strip and will most likely be darkened (especially in the case of greater film thicknesses).
Second, the part may be reverse plated electrolytically by switching the electrodes (i.e., making the part anodic) and proceeding at the same current density. This method will deplate the colors in spectral sequence, and the colors will be of slightly different appearance than the corresponding colors of the cathodically deposited color sequence. This method offers the most dramatic differences and may be useful for colored parts which have had a stop-off pattern applied to them.
Third, the part may be reduced electrolytically using a caustic pH or neutral bath of sodium or potassium hydroxide, carbonate or lactate. Another method to accomplish this is advantageous for the practitioner. An existing electrolytic mild caustic electroplating cleaner may be used without changing the polarity of the electrodes. The part is reduced at 4 to 6 volts D.C. using xe2x80x9cdirect hook-up,xe2x80x9d i.e., the part is cathodic. At this voltage, the colored coating will also lighten or become color-altered. This change in color tone will be more noticeable if reduced in a hot solution.
A supremely important embodiment from a commercial perspective is the discovery that by interrupting the current for a period of 20 seconds to 2:00 minutes and then resuming electrolytic deposition, the article can, for unknown reasons, actually achieve a desired color in less time than if the power interruption method had not been used. Other modifications, such as pulse plating and reverse pulse plating, are obvious equivalents.
It is important from a commercial vantage to be able to coat mass produced articles of consistent color. Therefore, it is necessary to match a color sample. We have discovered techniques to improve the ability of the operator to match colored coating.
A primary object of the invention is to provide for the coloring of articles for both decorative and functional purposes by means of utilizing light polarizing thin films of nickel and nickel alloy oxides on electroplated or mechanically surface finished articles whereby the undercoating may be Ti, Co, Ni, Cu, and such alloys as stainless steels, brasses and bronzes, steel, etc.
An object of the invention is to provide a color electroplated coating and process therefor which is capable of producing, and reproducing with exactitude, the deposition of various colors and hues through the utilization of light polarizing nickel oxide or nickel-zinc oxide electro-deposition baths without altering the chemical composition of the electrolyte in order to achieve said colors.
A further object of the invention is to provide a glass-like or vitreous appearance to the colored metallic thin-film and to provide a sense of binocular depth to the observer of the color plated article. An accompanying object of the invention is to provide a thin film colored coating with superior adhesion characteristics.
A still further object of the invention is to provide a thin film colored coating which is resistant to degradation by ultraviolet light.
It is a desired object of the invention to provide a coating which can be deposited on metallized plastics (e.g., polyvinyl chloride), electroconductive glasses and ceramics, as well as electroconductive metallic seed layers.
Another object of the invention is to deposit such light polarizing thin films on electroconductive transparent substrates, such as glass, glass fibers, including an indium tin oxide (ITO), coated polyester, such as polyethylene terephthalate, or a conductive transparent substrate, such as polyaniline (emeraldine).
It is an object of the invention to deposit colorful films of thin films of nickel and nickel alloy oxides on electroconductive ceramics.
Still another object of the invention is to provide an environmentally friendly electrodeposition process for depositing such colored coatings.
An object of the invention is to provide a bath electrolyte which is free of noxious odors and related occupational safety and health concerns.
Yet a further object of the invention is to provide a colored overlay for zinc electroplated articles and, by using neutral or high pH embodiments of the bath, producing a color film coating to be deposited directly over raw zinc and aluminum die castings.
An object of the invention is to provide a bath process for the controlled electrodeposition of metallochromy nickel zinc oxides on an article by altering the means of deposition, e.g., direct current, reverse current, altering the current density and using in situ oxidation by selective current interruption.
A further object of the invention is to provide a thin-film colored coating which is transparent at a certain range of thicknesses and which resembles allochromatic or opaque colors at greater thicknesses.
A further object of the invention is to provide a thin-film colored coating which is resistant to reduction or degradation at high temperatures.
Other objects of the invention will become apparent through disclosure in the specification hereinbelow.